Image forming device and image processing system

ABSTRACT

For obtaining a natural image with certainly providing visibility upon printing, an image forming apparatus includes a data buffer temporarily storing a printing data input through an input terminal, a printing data converting portion active when the description of a character is contained data temporarily stored in the data buffer, for converting the printing data for enhancing visibility of the character, an output terminal for externally outputting the printing data after conversion and a process end signal output terminal for externally outputting a signal when end of printing data is detected by a data end judgment portion.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to an image forming apparatus for improving degradation of visibility of printing product upon printing on a paper. More particularly, the invention relates to an image forming apparatus useful for monochrome printing of a color image or a gray scale image.

Description of the Related Art

[0002] Conventionally, in Japanese Patent Application Laid-Open No. Heisei 5 (1993)-250113, there has been disclosed a technology for obtaining conspicuous monochrome image upon monochrome printing of an image, in which color image data and code data are mixedly present by outlining an image around a portion where an image data and a code data overlap.

[0003] When the image around the portion where the image data and the code data overlap, is outlined, a gap to an original image becomes large to be unnatural image.

SUMMARY OF THE INVENTION

[0004] It is therefore an object of the present invention to obtain a natural image with certainly providing visibility, upon printing.

[0005] According to the first aspect of the present invention, an image forming apparatus comprises:

[0006] a data buffer storing an input printing data;

[0007] a judgment portion for making judgment whether a define statement of a predetermined character is included in data stored in the data buffer; and

[0008] a line screen value converting portion for converting a line screen definition for performing halftone printing of the predetermined character when a define statement of the predetermined character is contained in data stored in the data buffer.

[0009] In the preferred construction, the line screen value converting portion may comprise:

[0010] a first information input terminal for receiving input of a value of line screen to be converted;

[0011] a second information input terminal receiving an input designating a font size to be conversion object;

[0012] a font size comparing portion active when a description of the font size in the data in the data buffer, for comparing the font size with a font size given from the second information input terminal;

[0013] a line screen conversion character string storage portion storing an additional character string performing line screen conversion; and

[0014] a selector for switching output between input printing data and the line screen conversion character string on the basis of result of judgment in the character string judgment portion and comparison in the font size comparing portion.

[0015] The line screen value converting portion may comprise:

[0016] a background color information extracting portion for extracting information of a background color from the printing data; and

[0017] a line screen value determining portion for determining a line screen value for conversion with taking the extracted background color into account.

[0018] According to the second aspect of the present invention an image forming apparatus comprises:

[0019] a data buffer storing an input printing data;

[0020] a character string judgment portion for making judgment whether a define statement of a predetermined character in data stored in the data buffer; and

[0021] a color information converting portion for converting a color definition of the character to be printed.

[0022] In the preferred construction, the color information converting portion may comprise:

[0023] a color judgment portion for extracting color information in the data in the data buffer and making judgment whether degradation of visibility can be caused upon gray scale conversion;

[0024] a color conversion character string storing portion storing the character string to perform color conversion; and

[0025] a selector for switching output between the input printing data and the character string with converted color definition on the basis of result of judgment in the character string judgment portion and comparison in the font size comparing portion.

[0026] The color information converting portion may comprise:

[0027] a background color information extracting portion for extracting information of the background color from the printing data; and

[0028] a color information determining portion for determining color information to be converted on the basis of extracted background color information.

[0029] According to the third aspect of the present invention, an image forming apparatus comprises:

[0030] a data buffer temporarily storing a printing data input through an input terminal;

[0031] a printing data converting portion active when the description of a character is contained data temporarily stored in the data buffer, for converting the printing data for enhancing visibility of the character;

[0032] an output terminal for externally outputting the printing data after conversion; and

[0033] a process end signal output terminal for externally outputting a signal when end of printing data is detected by a data end judgment portion.

[0034] The printing data converting portion may comprise:

[0035] a first information input terminal for receiving input of a value of line screen to be converted;

[0036] a second information input terminal receiving an input designating a font size to be conversion object;

[0037] a font size comparing portion active when a description of the font size in the data in the data buffer, for comparing the font size with a font size given from the second information input terminal;

[0038] a line screen conversion character string storage portion storing an additional character string performing line screen conversion; and

[0039] a selector for switching output between input printing data and the line screen conversion character string on the basis of result of judgment in the character string judgment portion and comparison in the font size comparing portion.

[0040] In such case, the line screen value converting portion may comprise:

[0041] a background color information extracting portion for extracting information of a background color from the printing data; and

[0042] a line screen value determining portion for determining a line screen value for conversion with taking the extracted background color into account.

[0043] In the alternative, the printing data converting portion may comprise:

[0044] a color judgment portion for extracting color information in the data in the data buffer and making judgment whether degradation of visibility can be caused upon gray scale conversion;

[0045] a color conversion character string storing portion storing the character string to perform color conversion; and

[0046] a selector for switching output between the input printing data and the character string with converted color definition on the basis of result of judgment in the character string judgment portion and comparison in the font size comparing portion.

[0047] The color information converting portion may comprise:

[0048] a background color information extracting portion for extracting information of the background color from the printing data; and

[0049] a color information determining portion for determining color information to be converted on the basis of extracted background color information.

[0050] According to the fourth aspect of the present invention, an image processing system comprises:

[0051] a printer driver converting input image data into a printing data; and

[0052] a printer apparatus performing data output using the converted image data,

[0053] the printer driver including at least the image forming apparatus set forth above.

[0054] According to the fifth aspect of the present invention, an image processing system comprises:

[0055] a printer driver converting input image data into a printing data; and

[0056] a printer apparatus performing data output using the converted image data,

[0057] the printer apparatus including at least the image forming apparatus as set forth above.

[0058] According to the sixth aspect of the present invention, an image forming method may comprise:

[0059] parsing process for parsing information of an input printing data;

[0060] converting process for converting the printing data based on the parsing process; and

[0061] output process for outputting on the basis of printing data process through the conversion process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be imitative to the invention, but are for explanation and understanding only.

[0063] In the drawings:

[0064]FIG. 1 is a block diagram showing a construction of the first embodiment of an image forming apparatus according to the present invention;

[0065]FIG. 2 is a block diagram showing a construction of a line screen converting portion in the first embodiment of the image forming apparatus according to the present invention;

[0066]FIGS. 3A, 3B and 3C are explanatory illustrations showing examples of system construction of image processing system to be employed in the image forming apparatus according to the present invention;

[0067]FIG. 4 is an explanatory illustration showing a process flowchart in accordance with the present invention;

[0068]FIG. 5 is a block diagram showing a construction of the second embodiment of an image forming apparatus according to the present invention;

[0069]FIG. 6 is an explanatory illustration showing an effect by increasing of line screen in the present invention;

[0070]FIG. 7 is a block diagram showing a construction of a color information converting portion in the second embodiment of the image forming apparatus according to the present invention; and

[0071]FIG. 8 is a block diagram showing a construction of the third embodiment of an image forming apparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0072] The present invention will be discussed hereinafter in detail in terms of the preferred embodiments of image forming apparatus and image processing system according to the present invention with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structure are not shown in detail in order to avoid unnecessary obscurity of the present invention.

[0073] Upon outputting data containing color information by a monochrome printer, a conversion process for converting the color information into a gray scale is performed. Particularly, among color information for each pixel, brightness information is extracted to convert into a gray scale level between white˜gray˜black on the basis of the extracted brightness information. Upon practically outputting a gray scale image by the monochrome printer, the gray scale level has to be expressed by a binary value of white and black. For this reason, halftone dot called as halftone cell is employed to realize pseudo gray scale expression. For example, when a size of one halftone cell is sixteen dot square and when output resolution of printing output is 2400 dpi, a hundred-fifty halftone cells are aligned in one inch. Number of the halftone cells will be hereinafter referred to as line screen which represents number of lines of dots appearing per linear inch. Once the line screen and resolution are determined, size of the halftone cell is determined. On the other hand, depending upon size of the halftone cell, capability of gradation expression is determined. A ratio of white dots and black dots in the halftone cell expresses gray scale level of the halftone dot determines number of gradation levels in gray scale expression to be expressed by the halftone cell. For example, when the size of one halftone cell is constructed by sixteen dots square, two-hundreds fifty-six gradation levels can be expressed. In general, a relationship of

Number of gradation levels=(resolution(dpi))/(line screen(lpi))⁻2)

[0074] is established. Due to the foregoing relationship between the line screen, the resolution and number of gradation levels, smaller value of line screen achieves greater number of gradation levels to be expressed. A monochrome expression to be expressed through the foregoing conversion process for an original color image can be converted into expression to cause degradation of visibility in some objective original image. The present invention has been worked out for solving the problem set forth above.

[0075] On the other hand, in general, when the image data such as photographic image is to be printed, greater number of gradation levels achieved higher expression ability of the gray scale to achieve more natural feeling. On the other hand, upon printing data containing monotone data, such as character, greater line screen increases clarity of expression in detailed portion, such as edge or the like to be more conspicuous. However, upon practically printing by the printer, the resolution, the line screen are constant at fixed value in the image data to be printing object. Therefore, number of gradation levels naturally becomes constant. When the line screen is set small with giving higher preference for the image data, visibility of the color character data, particularly small character is sacrificed. Conversely, when the line screen is set large with giving higher preference for the character data, number of gradation levels of the image expression is reduced to make the printed image expressionlessly crushed as a whole. The present invention has been worked out for presenting solution for the problems set forth above.

[0076] On the other hand, it should be noted that the object of the present invention should not be limited to the monochrome printer but is also applicable for color printer output for providing solution for degradation of visibility to be caused by coloration of the background color and graphic pattern and/or character. The present invention has been worked out with focusing the drawbacks set forth above and thus to provide means for optimally setting printing characteristics for objective image data and character data by converting only portions possibly causing degradation of visibility with inserting a predetermined setting character string in a printing data. Hereinafter, the preferred embodiments of the present invention will be discussed hereinafter.

[0077] (First Embodiment)

[0078]FIG. 1 is a schematic block diagram of the first embodiment of an image forming apparatus according to the present invention, which can perform resolution conversion. The shown embodiment of the image forming apparatus generally comprises the following components 1 to 7. The reference numeral 1 denotes an input terminal, through which a printing data is to be input. In the shown embodiment, a postscript data described in a printer description language is an input objective data. The reference numeral 2 denotes a data buffer accumulating input printing data per one line. The data buffer stores the input data with shifting per one line. The reference numeral 3 denotes a data end judgment portion for making judgment whether the data in the data buffer 2 is the end data among data stored in the data buffer 2. Namely, the data end judgment portion 3 typically detects the end of stored data by detecting “EOF” indicative end of the file. The reference numeral 4 denotes a character judgment portion which makes judgment whether a define statement of a particular character. The reference numeral 5 denotes a line screen value converting portion for converting a line screen definition for converting a character into halftone data to be printed when a description of the character is contained in the stored data in the data buffer 2. The reference numeral 6 denotes an output terminal for externally outputting the printing data after performing line screen conversion. The reference numeral 7 denotes a process end signal output terminal for externally outputting a signal upon detection of end of fine by the data end judgment portion 3.

[0079]FIG. 2 shows a particular embodiment of the line screen value converting portion 5. The reference numeral 21 denotes an information input terminal for inputting a value of line screen to be converted. The reference numeral 22 denotes an information input terminal designating a font size to be conversion object. The reference numeral 23 denotes a font size comparing portion for comparing the font size given from the information input terminal 21. The reference numeral 24 denotes a line screen conversion character string storage portion storing an additional character string for performing line screen conversion. The reference numeral 25 denotes a selector for selectively switching the input data and the line screen conversion character string to output on the basis of the result of the character string judgment portion 4 and the font size comparing portion 23.

[0080] The first embodiment of the image forming apparatus takes the printing data as input. The image forming apparatus for improving visibility of printing output by parsing information of the printing data, comprises the input terminal for inputting the printing data, the data buffer 2 accumulating the printing data per one line, the data end judgment portion 3 for making judgment whether the data in the data buffer is end of the printing data, the character judgment portion 4 for making judgment whether the define statement of the predetermined character is contained in data in the data buffer 2, the line screen value converting portion 5 for converting line screen definition for converting the character to be printed into halftone data, the output terminal 6 for externally output the printing data after conversion and the process end signal output terminal 7 for externally outputting the signal upon detection of end of the file by the data end judgment portion 3. The line screen value converting portion 5 comprises the first information input terminal 21 for inputting the value of the line screen to be converted, the second information input terminal 22 designating the font size to be the conversion object, the font size comparing portion 23 comparing the font size with the font size given from the information input terminal, the line screen conversion character string storage portion 24 for storing additional character string for performing line screen conversion, and the selector 25 selectively switching between the input printing data and the output of the line screen converting character string to output depending upon the result of judgment in the character string judgment portion 4 and comparison in the font size comparing portion 23.

[0081]FIG. 3 shows an example of construction of an image processing system to be employed in the image forming apparatus according to the present invention.

[0082] In FIGS. 3A, 3B and 3C, the reference numeral 31 denotes the image processing system typically represented by a personal computer. It is not required that the image processing system is not specified to be the personal computer but can be arithmetic processor, data accumulator or the like having terminal for outputting the printing data. The reference numeral 32 denotes the image forming apparatus shown in FIG. 1. The reference numeral 33 denotes the image output device, such as the printer, for outputting data on the paper on the basis of the printing data. In the shown embodiment, the image processing system 31 is desirably a monochrome output postscript printer which can parse and print the printer description language post script language.

[0083] In FIG. 3A, the image forming apparatus 32 is incorporated in the image processing system 31 for operation. In FIG. 3B, the image forming apparatus is incorporated in the image output device 33. In FIG. 3C, the image forming apparatus 32 is not incorporated neither in the image processing system 31 nor the image output device 33 and is located therebetween to permit independent operation. The image processing system shown in FIG. 3 is used as a printing system, for example.

[0084] It should be noted that the function of the image forming apparatus 32 may be realized by hardware or by software. Accordingly, the function of the image forming apparatus may be realized by the software as program to be executed on the image processing system 31 or the image output device 33.

[0085]FIG. 4 is one embodiment of a process flowchart of the present invention. It should be noted that the shown embodiment is premised to the postscript data as the input data, the present invention is applicable for printing other type printing data.

[0086] At first, a printing data 400 obtained from the input terminal 1 of FIG. 1 is stored in the data buffer 2 per one line (step 401). In the character judgment portion 4, judgment is made whether the printing object contains a character string as the character data (step 402). The printing data in postscript description frequently differentiate definition of the character string per application. When define statement of the character string indicative of the character data is “MF”, and when this character string is expressed in one line of the character data, it indicates that character information, such as font size, fond kind and so forth is described in the line. Therefore, the character judgment portion 4 may be set for detecting this character string “MF”. When the character string is detected (YES in step 402), judgment is made that the printing object is the character data to extract the font size of the character to print from the relevant line in the fond size comparing portion 23. Position of the description of the font size is fixed by the software as source of generation of the character data, and thus can be easily extracted.

[0087] For example, in the shown embodiment, when the font definition character string in the printing data is [300 0 0−300 0 0]/AATT11E17C23A8-0 MF, it is premised that the numerical value “300” is the value determining the font size, and the character string “/AATT11E17C23A8-0” expresses the kind of font.

[0088] Comparison is made between the extracted font size and the font size input from the information input terminal 21 (step 403). When the font size of the character to print is smaller (YES in step 403), a command is output to the selector for outputting the character string in the line screen conversion character string storage portion 24. In practice, a description to increase line screen is added (step 404). In the selector 25, the data in the data buffer 2 is normally output. However, when judgment is made in the character judgment portion 4 and the font size comparing portion 23 that the line screen conversion character string is to be output, outputting of data of the data buffer 2 is temporarily stopped to switch the data path for outputting the character string in the line screen conversion character string storage portion 24. As a result, data produced by inserting the line screen conversion character string to the input printing data can be output from the output terminal 6.

[0089] When the operation is repeated per one line of the input data and when the character string indicative of the end of data is detected in the data buffer 2 in the data end judgment portion 3 (YES in step 405), the shown process goes END to externally output a signal indicative of end of process from the output terminal 7. In response to this, new printing data 406 is prepared. As alternative method of data end judgment other than that detecting the character string indicative of the end of data as set forth above, it is possible to make judgment of data end when data amount of the input data reaches a predetermined amount. In the further alternative, it is possible to make judgment when a predetermined period is elapsed after input. In short, any method which may detect possible data end, may be used.

[0090] The character string in the line screen conversion character string storage portion 24 may be formed in accordance with description of the postscript. The following is one example of the character string: /freq X def /HProc {dup mul exch dup mul add I exch sub} def freq 45 /HProc load setscreen

[0091] In the character string, X is line screen value to be converted and is replaced with a line screen designation parameter obtained from the information. Greater value of X may improve visibility of the character.

[0092]FIGS. 6A and 6B are imaginary illustrations showing the effect of improvement of visibility by increasing of the line screen. In FIGS. 6A and 6B, each circle represents dot to be printed. One inch length is assumed as shown in FIGS. 6A and 6B, the resolution of the shown figure becomes 20 dpi. FIG. 6A shows the character expressed by a halftone cell of 2×2 dots. In this case, gradation level is 4, and line screen is 10 lpi. On the other hand, in FIG. 6B, the halftone cell is formed by single dot. Therefore, the gradation level is 2 and the line screen is 20 lpi. If the original character is colored, the gradation expression of the character is done by halftone expression. In these Figures, there is shown an example of expression of gradation by making two dots in the four dot halftone cell in FIG. 6A. On the other hand, in the example of FIG. 6B, since only two tone expression is possible, the cells located in the character portion is expressed by black irrespective of the original color. From comparison of FIGS. 6A and 6B, it should be clear that visibility of FIG. 6B having greater line screen is much higher than that of FIG. 6A.

[0093] (Second Embodiment)

[0094]FIG. 7 is a schematic block diagram showing a construction of the second embodiment of the image forming apparatus performing resolution conversion according to the present invention. The shown embodiment of the image forming apparatus is constructed with the following components identified by the reference numerals 1 to 7 and 70. The components identified by the reference numerals 1 to 7 are the same as those discussed in connection with the first embodiment. The reference numeral 70 denotes a color information converting portion converting color definition for printing the character when data in the data buffer 2 contains description of the character.

[0095]FIG. 5 shows an alternative embodiment of particular example for realizing the color information converting portion 70. The reference numeral 51 denotes a color judgment portion for extracting color information in the data stored in the data buffer 4 and making judgment whether degradation of visibility can be caused upon conversion into gray scale image. The reference numeral 52 denotes a color conversion character string storage portion storing the character string to perform color conversion. The reference numeral 53 denotes a selector for switching output between the input printing data and the color conversion character string depending upon the result of judgment in the character string judgment portion 4 and the font size comparing portion 23. A system construction is similar to FIG. 3.

[0096] At first, the printing data obtained from the input terminal 1 of FIG. 1 is temporarily stored in the data buffer 2 per one line. In the character string judgment portion 4, judgment is made whether a character string indicative of the character data is contained in the printing object or not. When the character string is detected and thus judgment is made that the printing object is the character data, color information of the character is extracted in the color judgment portion 51. If judgment is made that the color of the character is light as converted into gray scale, a command is output to the selected so that the character string in the color conversion character string storage portion 52 is out. In the selector 53, data in the data buffer 2 is normally output. However, when judgment is made that outputting of the color conversion character string is necessary, in the character judgment portion 4 and the color judgment portion 51, the selector is switched for interrupting outputting of the data in the data buffer 2 as done in normal state, and outputting the character string in the color conversion character string storage portion 52. As a result, from the output terminal 6, the color information from the input printing data can be converted into the color information having high visibility. When the operation is repeated per one line of the input data and when the character string indicative of the end of data is detected in the data buffer 2 in the data end judgment portion 3, the shown process goes END to externally output a signal indicative of end of process from the output terminal 7.

[0097] The character string of the color conversion character string storage portion 24 can improve visibility of the character by describing to make the character to be expressed in black, uniformly.

[0098] (Third Embodiment)

[0099] In the present invention, in the preferred embodiment, line screen value conversion and/or color conversion may be performed with taking the background color of the original into consideration. FIG. 8 is a schematic block diagram showing the third embodiment of the image forming apparatus according to the present invention permitting printing in consideration of the background color of the original.

[0100] In FIG. 8, the components identified by the reference numerals 1 to 7, 21, 23 to 25 are similar to those discussed in connection with the first embodiment. The reference numeral 80 denotes a background color information extracting portion extracting a background color information in the printing data description stored in the data buffer. The reference numeral 81 denotes a line screen value determining portion for determining a line screen for converting in consideration of the extracted background color. While the second embodiment determines the line screen value derived by external input, the shown embodiment determines the line screen value in consideration of the background color of the printing data in the background color information extracting portion 80 and the line screen value determining portion 81. Other operations are the same as those in the first embodiment. Also, the shown embodiment may be modified by adding the similar function to the color information converting portion 70 n in the second embodiment, color information to convert is determined.

[0101] As set forth above, in the preferred embodiments of the present invention, by inserting setting character string in arbitrary portion in the printing data, means for setting of optical printing characteristics for the objective image data and the character data can be provided. Namely, the image forming apparatus receiving input of the printing data and improving visibility of the printing output with parsing information of the printing data includes the input terminal receiving the printing data as input, the data buffer storing the printing data per one line, the data end judgment portion for making judgment whether the data in the data buffer is end data in the printing data, the character string judgment portion for making judgment whether the define statement of the predetermined character in the data in the data buffer, the printing data converting portion converting the printing data for improving visibility of the character when the description of the character is contained in the data buffer, the output terminal for externally outputting the printing data after conversion, and the process end signal output terminal for externally outputting the signal when end of file is detected by the data end judgment portion. By adding or converting the character string for improving visibility of the character font smaller than or equal to a designated character size, expression of the detail portion, such as edge of the character image can be made clear. As set forth above, by adding the character string improving visibility of the character font smaller than or equal to the character size, expression of the detail portion, such as edge of the character image can be made clear.

[0102] Although the present invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omission and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalent thereof with respect to the feature set out in the appended claims.

[0103] For instance, while the embodiments have been discussed in terms of printing output of the color original by a monochrome printer, the present invention should not be limited to this. Namely, the present invention is applicable for the case where the color original is output by a color printer. 

What is claimed is:
 1. An image forming apparatus comprising: a data buffer storing an input printing data; a judgment portion for making judgment whether a define statement of a predetermined character is included in data stored in said data buffer; and a line screen value converting portion for converting a line screen definition for performing halftone printing of said predetermined character when a define statement of said predetermined character is contained in data stored in said data buffer.
 2. An image forming apparatus as set forth in claim 1, wherein said line screen value converting portion comprises: a first information input terminal for receiving input of a value of line screen to be converted; a second information input terminal receiving an input designating a font size to be conversion object; a font size comparing portion active when a description of the font size in the data in said data buffer, for comparing the font size with a font size given from said second information input terminal; a line screen conversion character string storage portion storing an additional character string performing line screen conversion; and a selector for switching output between input printing data and said line screen conversion character string on the basis of result of judgment in said character string judgment portion and comparison in said font size comparing portion.
 3. An image forming apparatus as set forth in claim 2, wherein said line screen value converting portion comprises: a background color information extracting portion for extracting information of a background color from the printing data; and a line screen value determining portion for determining a line screen value for conversion with taking the extracted background color into account.
 4. An image forming apparatus comprising: a data buffer storing an input printing data; a character string judgment portion for making judgment whether a define statement of a predetermined character in data stored in said data buffer; and a color information converting portion for converting a color definition of the character to be printed.
 5. Am image forming apparatus as set forth in claim 4, wherein said color information converting portion comprises: a color judgment portion for extracting color information in the data in said data buffer and making judgment whether degradation of visibility can be caused upon gray scale conversion; a color conversion character string storing portion storing the character string to perform color conversion; and a selector for switching output between the input printing data and the character string with converted color definition on the basis of result of judgment in said character string judgment portion and comparison in said font size comparing portion.
 6. An image forming apparatus as set forth in claim 5, wherein said color information converting portion comprises: a background color information extracting portion for extracting information of the background color from the printing data; and a color information determining portion for determining color information to be converted on the basis of extracted background color information.
 7. An image forming apparatus comprising: a data buffer temporarily storing a printing data input through an input terminal; a printing data converting portion active when the description of a character is contained data temporarily stored in said data buffer, for converting said printing data for enhancing visibility of the character; an output terminal for externally outputting the printing data after conversion; and a process end signal output terminal for externally outputting a signal when end of printing data is detected by a data end judgment portion.
 8. An image forming apparatus as set forth in claim 1, wherein said printing data converting portion comprises: a first information input terminal for receiving input of a value of line screen to be converted; a second information input terminal receiving an input designating a font size to be conversion object; a font size comparing portion active when a description of the font size in the data in said data buffer, for comparing the font size with a font size given from said second information input terminal; a line screen conversion character string storage portion storing an additional character string performing line screen conversion; and a selector for switching output between input printing data and said line screen conversion character string on the basis of result of judgment in said character string judgment portion and comparison in said font size comparing portion.
 9. An image forming apparatus as set forth in claim 8, wherein said line screen value converting portion comprises: a background color information extracting portion for extracting information of a background color from the printing data; and a line screen value determining portion for determining a line screen value for conversion with taking the extracted background color into account.
 10. Am image forming apparatus as set forth in claim 4, wherein said printing data converting portion comprises: a color judgment portion for extracting color information in the data in said data buffer and making judgment whether degradation of visibility can be caused upon gray scale conversion; a color conversion character string storing portion storing the character string to perform color conversion; and a selector for switching output between the input printing data and the character string with converted color definition on the basis of result of judgment in said character string judgment portion and comparison in said font size comparing portion.
 11. An image forming apparatus as set forth in claim 5, wherein said color information converting portion comprises: a background color information extracting portion for extracting information of the background color from the printing data; and a color information determining portion for determining color information to be converted on the basis of extracted background color information.
 12. An image processing system comprising: a printer driver converting input image data into a printing data; and a printer apparatus performing data output using the converted image data, said printer driver including at least said image forming apparatus defined in any one of claims 1 to
 11. 13. An image processing system comprising: a printer driver converting input image data into a printing data; and a printer apparatus performing data output using the converted image data, said printer apparatus including at least said image forming apparatus defined in any one of claims 1 to
 11. 14. An image forming method comprising: parsing process for parsing information of an input printing data; converting process for converting the printing data based on said parsing process; and output process for outputting on the basis of printing data process through said conversion process. 